Many animals, especially dogs and cats, are afflicted with a condition known as “eye stain” or “tear stain,” which is characterized by excessive discharge from the eyes that leaves an unsightly red or brown stain on the animal's hair. Such staining is especially pronounced in light-colored animals in which pigments in the tears can easily stain their hair.
Eye stain is thought to have a number of causes relating to excess tearing. The medical term chromodacryorrhea refers to an overproduction of tears with an excessive secretion of ferroporphyrins in tears, saliva, and urine. Ferroporphyrins have a characteristic red color and can cause eye stain particularly in light-colored animals. In mammals, ferroporphyrins are excreted primarily through bile and the intestinal tract. In dogs and cats ferroporphyrins are also excreted through tears, saliva and urine, and can stain hair in areas that come into contact with these bodily fluids.
Eye stain can also result from a variety of health, dietary, and genetic factors underlying excessive tearing. For instance, bacteria and yeast can grow on an animal's hair that is made moist and damp from excessive tearing. As a consequence, these bacteria and yeast grow in close proximity to the eyes and can cause infection of the eyes. Ptyrosporin, known as “red yeast,” is thought to be associated with reddish-brown facial stains and can cause a foul odor in the affected areas. Additionally, some animals can have shallow eye sockets, eyelids turned inward so as to block tear drainage holes, hair growth around the eye that wicks tears onto the face, or eye damage in which scar tissue blocks tear drainage holes. As such, there are multiple causes of excessive tearing responsible for eye stain in animals.